Tube amplifier troubleshooting

[maxhifi]

Originally Posted by drussell

Ahh, a fellow Albertan... We'll leave the rivalries to the sports teams. Actually, my brother lives in Edmonton now as his wife's family is from there and, uh... well... apparently he really loves her, or something... I'm not up there often but since it's always nice to meet a tube-appreciating kindred spirit, perhaps sometime we'll manage to get together for a

Absolutely, beer is always good.

Originally Posted by drussell

In other words, the fun part, right?

It's as if you know me... I like this amplifier exactly because it is weird, and I am not one to back down from a challenge. This hum issue nearly had me though, mainly because I kept looking under the wrong stone... it got louder with the input disconnected, so I couldn't let go of the fact it must be an issue with the input stage, when it clearly couldn't be because it was common to both channels. I really need to listen to myself sometimes, there's just no way a problem with an input stage would magically happen on both channels at once, when they are entirely separate and on different boards.

Originally Posted by drussell

One PAIR is supposed to be able to do 100W class A. The Mk.6 uses four to do 120W

Are you sure about that?

I have always been impressed by the Dynaco MK6, there's a few vintage amps I would really like to own, and that is one of them. Others are the Heathkit W6M, and the HK Citation-II. The tube amp you helped me to troubleshoot uses a very odd output tube, the 6R3S.. it's kind of like a double 807, with a 7 pin base, and twin plate caps. When I got the amplifier I was pouring through tube manuals, trying to find the closest American tube, without any success. There's just nothing even close which is even slightly easy to find, and like you I don't like modifying equipment, not that I couldn't but it spoils the original character, I would rather build from scratch than modify existing.

[drussell]

Originally Posted by maxhifi

Are you sure about that?

No... I'm not... It could well have been class AB, I'd have to find a databook and see what the ratings were but I thought there was some Bogen model or something that did 100W with a single pair but don't know how they were biased and it's been many years since that stuff was at the top of my head.

IIRC there were some Mcintosh and others that did 120W+ with four like the Dynaco so it may well be that you can only do 60-70W in class A. I also recall there was a Black Cat linear RF amp that could do 100W with one tube but that was class B or C or something for RF only, obviously, but I thought they had some silly plate dissipation rating like 50W and ridiculously high maximum plate voltage spec but I could very well be mistaken, it's been a long time.

I'll have to dig out one of the databooks later when I have time.

A very nice tube type, regardless.

[maxhifi]

Originally Posted by drussell

No... I'm not... It could well have been class AB, I'd have to find a databook and see what the ratings were but I thought there was some Bogen model or something that did 100W with a single pair but don't know how they were biased and it's been many years since that stuff was at the top of my head.

IIRC there were some Mcintosh and others that did 120W+ with four like the Dynaco so it may well be that you can only do 60-70W in class A. I also recall there was a Black Cat linear RF amp that could do 100W with one tube but that was class B or C or something for RF only, obviously, but I thought they had some silly plate dissipation rating like 50W and ridiculously high maximum plate voltage spec but I could very well be mistaken, it's been a long time.

I'll have to dig out one of the databooks later when I have time.

A very nice tube type, regardless.

It's class AB for sure, just giving a fellow tube guy a hard time
The EL34 apparently can also do 100W from a single pair, with 800V on the plate, but I would sure hate to see how this would work out with modern tubes!

[Zenith26kc20]

The metal based EL34 is the only one I know that can take the 800+ plate volts. I built an amp years ago and stupidly believed the tube manual maximum ratings. I couldn't get more than 80 watts per pair out of the metal based before boom! The regular EL34 tubes would turn blueish purple about 4 seconds after turn on and blow out the screen and cathode resistors.
I would say the most stable amps I have are a pair of Heath W6M amplifiers. The cathode follower driver stage makes it possible to use almost any 6550/KT88 tube as it prevents runaway.
I think chopping up Dyna MKVI to use other output tubes is a bit crazy. Have a look at the price for them and it becomes obvious!
My Quatre gain cells are definitly not going to win any award for stability

[maxhifi]

Originally Posted by Zenith26kc20

I would say the most stable amps I have are a pair of Heath W6M amplifiers. The cathode follower driver stage makes it possible to use almost any 6550/KT88 tube as it prevents runaway.

Man, Heath W6M... that's been one of my "dream amps" for years, how do you like them? Are they as good in reality as the schematic and specs would have someone believe?

[powerking]

I too (at one time 20 or so years ago; pre-Internet & eBay) was desiring a pair of W-6M's. Back then they were rare as hen's teeth. The go to "rag" for used equipment was AudioMart; it came out monthly. So I settled for the Citation-II (which I never fixed/restored). Another very good amp (and RARE) is the Regency HF-350A. I'm kicking myself for selling it in '88 for $75.

https://www.google.com/search?hl=en&...ml%3B500%3B319

Tom (PK)

[Zenith26kc20]

The W6M amps are, in my opinion the best sounding tube amps I've heard. They beat my Citation II and my Dyna MKVI amps. I've been repairing and collecting tube amps for most of my life. The W6M amps also can take a pretty good amount of abuse. My "better" half likes the music loud and the Heathkits could take it. The Citation II would burn out the screen grid resistors after a while. I never let her at the MKVI's because they have all original output tubes except one.
Now I use the DG500 Quatre for a daily driver. And yes, there are added fuses on the speaker wires in case it decides to have a temper tantrum.
I always loved the driver stage of the W6M amps. Whoever designed it must have known the 6550's bias creep issue!

[powerking]

I always thought that Heathkit's tube power amps were the immortal Reg Williamson designs (did he contract them to Heath back in the day)? Hence the model numbering W-xM designation (W=Williamson design, x=design generation/type, M=monoblock).

http://en.wikipedia.org/wiki/Williamson_amplifier

http://www.heathkit-museum.com/hifi/hvmw-6m.shtml

Tom (PK)

[Zenith26kc20]

The front end is classic Williamson. The fun part is the drive stage. Each 6550 has it's own direct coupled cathode follower. The bias is set by the grid of the cathode follower and the cathode is connected to a negative voltage source. The cathode is also connected to the 6550 grid. Also, the power transformer only gets barely warm after being on all day! Fun amplifiers!

[maxhifi]

Originally Posted by Zenith26kc20

The front end is classic Williamson. The fun part is the drive stage. Each 6550 has it's own direct coupled cathode follower. The bias is set by the grid of the cathode follower and the cathode is connected to a negative voltage source. The cathode is also connected to the 6550 grid. Also, the power transformer only gets barely warm after being on all day! Fun amplifiers!

I wonder if those cathode followers can drive the 6550s into AB2 at all.

As an update to the original thread, my amplifier is still working perfectly, no more hum.

[Zenith26kc20]

Took a minute to look at the 6RS3 tubes. The picture shows two per side on the amplifier. In actuality this treats them as four total per output stage? Looked the tubes up and they are very different looking. I remember a customer bringing in a Mc240 with 5932 output tubes that had double sections inside but no seperate connections for the two parts. Some websites call it a ruggedized 6L6. Does the amplifier use the double tube in parallel (treats each one (double insidews) as one tube)?

[maxhifi]

Originally Posted by Zenith26kc20

Took a minute to look at the 6RS3 tubes. The picture shows two per side on the amplifier. In actuality this treats them as four total per output stage? Looked the tubes up and they are very different looking. I remember a customer bringing in a Mc240 with 5932 output tubes that had double sections inside but no seperate connections for the two parts. Some websites call it a ruggedized 6L6. Does the amplifier use the double tube in parallel (treats each one (double insidews) as one tube)?

So I did a lot of research about this tube, when I first got the amplifier, with a strong desire to replace them with American tubes for reliability. From what I can tell, the base and plate cap arrangement is borrowed/adapted from dual beam power transmitting tubes such as the RCA 829B, but the tube characteristics are adapted for use in an audio amplifier. This was a common medium power output tube in soviet made sound equipment - PA amplifiers, and I think even "wired radio" amplifiers, but I have not fully confirmed that fact. It actually packs a big punch into a little space, I would call it roughly equivalent to a 6550 in terms of ability to make power, but certainly not in terms of good looks or safety

The amplifier runs the tubes in push pull parallel, with the two units inside each tube connected in parallel.

What makes this tube especially hard to find a domestic substitute for, aside from the totally weird socket and cap arrangement, is the low impedance, and also just how much power it packs into a small package. The output stage operates as follows:

Plate voltage: 480
Screen grid: 240
Cathode current at idle: 65mA per pair
Grid voltage: approx. -32V
Output transformer load: approx. 2600 ohms plate to plate for the parallel pair.
Power output: approx. 80W

The best I could come up with for a replacement, would be to raise the screen voltage slightly and use FOUR 807s per side, in push pull parallel . Of course this isn't too practical, so I just stuck with the 6R3S. Using 829Bs is not possible either; the socket is different (one fat pin instead of a keyed envelope), and the tube requires a higher impedance output transformer.

I think that it MAY be possible to lower the screen voltage a bit, and use a pair of big horizontal output tubes per side, but in this case I am substituting one rare tube for another, and not a lot is gained.

As it sits, the amplifier sounds very good and is stable, I use it for hours every day. But during the process of getting it going, I encountered several 6R3S which would run away and red plate after 20 minutes. It took 7 tubes to make a good set of four, and hopefully the ones I have now will last. The good thing is, these tubes are not particularly rare or expensive, because they have very limited applications in 2014.

Edit: The common modification for this amplifier, for Russians, involves replacing the 6R3S with triode connected GU-50 tubes (same socket, and losing the plate caps). I personally hate this idea, I didn't go to all this trouble of getting a powerful amplifier, just to turn it into a 12W triode amplifier... I like having 75 Watts, it's so much more fun to have seemingly limitless tube power, which pushes the cones of my old Linns well into the non linear region without even breaking a sweat.